The present invention relates to superparamagnetic ferrofluid materials and the use thereof in magnetic refrigeration processes. More particularly, the invention relates to magnetic refrigeration compositions and processes for use in magnetic refrigeration processes and which processes are suitable for use in magnetic refrigerators having a refrigeration initiating temperature above about 273.degree. K. This invention is particularly applicable to magnetic heating and cooling systems and compositions which comprise nanocomposite superparamagnetic ferrofluid materials.
Magnetic refrigeration is a type of refrigeration which is accomplished generally by the cyclic operations of heat dissipation and heat absorption in the course of magnetization or demagnetization by adding or eliminating external magnetic fields applied to a magnetic substance. Magnetic refrigeration processes are theoretically analogous to conventional refrigeration processes which are accomplished by means of cyclic compression and expansion of gaseous systems. The use of magnetic fluids or ferrofluids as a coolant is disclosed, for example, in Magnetic Fluids Guidebook: Properties and Applications, V. E. Fertman, Hemisphere Publishing Corp., New York, 1990.
Magnetic refrigeration systems have been principally developed and used for achieving very low temperatures, for example in the milliKelvin range, for the purpose of investigating the physics of materials at low temperatures. These very low temperatures were achieved using, for example, an adiabatic magnetization and demagnetization process sequence with, for example, a paramagnetic salt, wherein the salt is magnetized during a first stage of cooling to low temperatures, by external means, followed by an adiabatic demagnetization of the salt to achieve a second stage of cooling and very low temperatures.
In low temperature regions, such as below about 15.degree. K, the lattice specific heat of magnetic materials for magnetic refrigeration becomes small compared to the magnetic specific heat, and a large magnetic entropy change may occur. The lattice specific heat values are negligible so that magnetic refrigeration can be accomplished in low temperature regions by using a magnetic refrigeration cycle of the reverse Carnot type. Paramagnetic substances are suitable materials for use in magnetic refrigeration in this low temperature region, especially in the region below about 20.degree. K, for example gadolinium gallium garnet.
In higher temperature regions, for example at or above 77.degree. K, the aforementioned lattice specific heat of the magnetic substance generally becomes larger than the magnetic specific heat so that a reverse Carnot type cycle is not usable and an Ericsson cycle and ferromagnetic and ferrimagnetic materials must be used. Furthermore, the heat disturbance energy of the magnetic moment also becomes greater for paramagnetic materials at these temperature and thus paramagnetic materials become unsuitable refrigerants at these temperatures.
Magnetic refrigeration processes possess inherently high entropy changes during reversible magnetic ordering and disordering. On a volumetric basis it is estimated that magnetic refrigerators may possess entropy changes of from about 40 to about 200 times greater than those observed for compressible gas refrigeration systems.
When an external magnetic field is applied to a material, the magnetic spins in the material attempt to align with the magnetic field, thereby reducing the magnetic entropy of the spin system. If this process is performed adiabatically, the reduction in spin entropy is offset by an increase in lattice entropy, and the temperature of the specimen will rise. This temperature rise, is reversible and is known as the magnetocaloric effect (.DELTA.T). Thus, magnetic entropy reduction (.DELTA.S) of a ferromagnetic or paramagnetic material upon adiabatic application of a magnetic field causes the lattice entropy to increase by the same amount (.DELTA.S), that is, the temperature of the system will increase by .DELTA.T+.DELTA.S/c, where c is the thermal capacity of the material at a constant applied magnetic field. Although the change of the magnetization in paramagnetic salts is very low, the small thermal capacity of these materials at low temperatures produces a surprisingly large magnetocaloric effect below about 1.degree. K. However, the inconvenience and energy required to attain and maintain the near absolute zero temperature and high magnetic field conditions are considerable, thermodynamically inefficient, and impractical for a variety of applications.
The reversibility of the aforementioned magnetic refrigeration process suggests that these processes could produce thermodynamic efficiencies approaching those of a Carnot cycle.
R. D. Shull et al., have described nanocomposite magnetic materials and there use in magnetic refrigeration, see for example, Nanostructured Materials, Vol. 1, 83-88, 1992, and Vol. 2, 205-211, 1993. However, these materials and systems are apparently limited in their application to refrigeration and are restricted to applications at or below about 25.degree. K. A particular composition suggested as a potential high temperature magnetic refrigerant was of the formula Nd.sub.0.14 (Fe.sub.1-x Al.sub.x) .sub.0.08 B.sub.0.06. For these or related materials to be useful in room temperature magnetocaloric applications, it is believed that the material must possess a Curie temperature of about 300.degree. K which they do not.
Two important classes of magnetic refrigerants are: paramagnetic materials for low temperature (T&lt;20.degree. K) systems and ferromagnetic systems for higher temperature operation, such as from about 20.degree. K to about 80.degree. K. Another new class of materials, known as magnetic nanocomposites, have been suggested as an alternative refrigerant for both high and low temperature regimes due to enhanced .DELTA.T values. It was also suggested that nanocomposite magnetic refrigerants might enable refrigerators to operate at reduced magnetic fields, see R. D. Shull et al., Nanostructured Materials, Vol. 2, 205-211, 1993.
U.S. Pat. No. 3,667,251, issued Jun. 6, 1972 to Miskolczy et al., discloses an absorption type refrigeration system in which a magnetocaloric pump system is used in combination with a compatible-ferrofluid refrigerant system to replace the percolator type pump or other conventional pumps.
U.S. Pat. No. 4,078,392, issued Mar. 14, 1978, to Kestner, discloses a direct contact refrigeration system utilizing magnetic fluids, sometimes referred to as ferrofluids, in combination with a suitable refrigerant. The ferrofluid is separated from the refrigerant by magnetic means and circulated to the cooling load. At the same time, the evaporated refrigerant is compressed, condensed and the expanded into direct contact with the warmer ferrofluid returning from the cooling load.
U.S. Pat. No. 4,704,871, issued Nov. 10, 1987, to Barclay et al., discloses a magnetic refrigerator operating in the 12.degree. to 77.degree. K range and utilizes a belt which carries ferromagnetic or paramagnetic material and which is disposed in a loop which passes through the center of a solenoidal magnet to achieve cooling. The magnetic material carried by the belt, which can be blocks in frames of a linked belt, can be a mixture of substances with different Curie temperatures arranged such that the Curie temperatures progressively increase from one edge of the belt to the other. This magnetic refrigerator can be used to cool and liquefy hydrogen or other fluids.
U.S. Pat. No. 4,956,976, issued Sep. 18, 1990, to Kral et al., disclosed is a magnetic refrigeration apparatus, modular in design, so that housing modules are alternately stacked with superconducting magnet pairs. Each module has a wheel that is rotated through the module, the wheel having cutout regions into which elements of magnetic material are inserted. Each cutout region has two elements separated by a wave spring, the wave spring biasing the elements against the housing module so that the elements are in slidable contact with the module upon rotation. In operation, the wheel carries the elements cyclically between high and low magnetic field zones. In low field regions the elements are cooled by the magnetocaloric effect and heat exchangers absorb heat from either a stagnant subcooled superfluid helium bath or a forced-flow subcooled superfluid helium stream. In high field regions the elements are heated by the magnetocaloric effect and a force flow stream of liquid helium passes through the high temperature heat exchangers absorbing heat from the magnetic refrigeration apparatus.
U.S. Pat. No. 5,231,834, issued Aug. 3, 1993, to Burnett, discloses a magnetic heating and cooling system. A magnetic fluid is pumped through at least a portion of the heating and cooling system. The fluid moves through the field of a superconducting or other type of magnet. When the fluid enters the magnetic field, it is heated as a result of the magnetization. Heat from the magnetic fluid is then transferred to a regenerator chamber. When the fluid leaves the magnetic field it is chilled. Heat from a regenerator chamber is then transferred to the fluid. External loads or sinks are heated or cooled.
U.S. Pat. No. 5,213,630, issued May 25, 1992, to Hashimoto, discloses a magnetic refrigeration composition for magnetic refrigeration including at least three kinds of magnetic substances selected from the group consisting of magnetic substances having the formula R'Al.sub.2, R'.sub.3 Al.sub.2, and R'Al.sub.2+delta, where in R' is at least one element selected from the group consisting of Gd, Tb, Dy, Ho, and Er, provided that the total number of atoms satisfies the above formula and 0&lt;delta&lt;0.2, the composition being a mixture of the magnetic substances, wherein each kind of said at least three kinds of magnetic substances has a Curie temperature which is different from that of the other kinds and which preferably range up to about 77.degree. K.
U.S. Pat. No. 5,182,914, issued Feb. 2, 1993, to Barclay et al., discloses a rotary dipole active magnetic regenerative refrigerator comprising a stationary first regenerative magnetic bed positioned with a stationary first inner dipole magnet, a stationary second regenerative magnetic material bed positioned with a stationary second inner dipole magnet, an outer dipole magnet that rotates on a longitudinal axis and encloses the inner dipole magnets, a cold heat exchanger, hot heat exchangers, a fluid displacer, and connective plumbing through which a heat transfer fluid is conveyed. The first and second regenerative magnetic beds are magnetized and demagnetized as the vector sums of the magnetic fields of the inner dipoles magnets and the outer dipole magnet are added together upon rotation of the outer dipole magnet, such magnetization and demagnetization causing a correlative increase and decrease in the temperature of the magnetic material beds by the magnetocaloric effect. Upon magnetization of any particular magnetic material bed, fluid flow is forced therethrough in the connective plumbing by the fluid displacer in the direction from the cold heat exchanger to one of the hot heat exchangers. Upon demagnetization of any particular magnetic material bed, fluid flow is reversed by the fluid displacer and is forced in the direction from one of the hot heat exchangers to the cold heat exchanger.
U.S. Pat. No. 5,316,699, issued May 31, 1994, to Ritter, Shull, et al., discloses a chemical process for producing bulk quantities of an iron-silica gel composite in which particle size, form, and magnetic state of the iron can be selected. The process involves polymerizing an ethanolic solution of tetraethylorthosilcate, ferric nitrate in water at low temperature under the influence of an HF catalyst. The chemical and magnetic states of the iron in the resultant composite are modified in situ by exposure to suitable oxidizing or reducing agent at temperatures under 400.degree. C. Iron-containing particles of less than 200 Angstroms diameter, homogeneously dispersed in silica matrices may be prepared in paramagnetic, superparamagnetic, ferrimagnetic and ferromagnetic states.
U.S. Pat. No. 4,238,558, discloses low density magnetic polymeric carrier materials containing a polymer material impregnated with a magnetic elemental metal or metal oxide derived from transition metal carbonyl compounds. According to the disclosure of this patent, the carrier particles are prepared by placing in a suitable vessel particles of a polymeric material, a suspending medium, and a transition metal carbonyl, heating the mixture with agitation for the purpose of thermally decomposing the transition metal carbonyl, causing the polymer to be impregnated with a magnetic elemental metal or metal oxide of a transition metal carbonyl, followed by cooling.
Moreover, there is disclosed in U.S. Pat. No. 4,150,173 a process for preparing transparent colored magnetic materials by, for example, heating a mixture of a silicaceous material, a suspending medium, and a transition metal carbonyl, wherein the silicaceous material is coated with a magnetic elemental metal of the transition metal carbonyl.
U.S. Pat. No. 4,474,866, assigned to the assignee of the present application, and which is incorporated herein by reference in its entirety, discloses a developer composition containing superparamagnetic polymers. The developer composition disclosed in this patent consists of a dispersion of fine particles of iron oxide in a polystyrene ion exchange resin. More specifically, the developer composition consists of .gamma.-Fe.sub.2 O.sub.3 (gamma) disposed in a sulfonated divinylbenzene cross-linked polystyrene resin.
The disclosures of each of the aforementioned documents are totally incorporated herein by reference.
In the aforementioned commonly assigned U.S. Pat. No. 5,362,417 (D/90063) there is disclosed a method of forming a colloidal dispersion of submicron particles comprising: providing an ion exchange resin matrix; loading said resin matrix with an ion; and treating the resin to cause in-situ formation of submicron particles; and fluidizing said ion exchange resin and particles in an aqueous medium to form a stable colloid of said particles.
In the aforementioned commonly assigned copending application U.S. Ser. No. 08/332,174 (D/94178) is disclosed a method for producing a magnetized pigment comprising the steps of: forming a magnetic material core in a vaporized state from a vaporized magnetic material; forming a pigment coating on the magnetic material core while in the vaporized state.
A long standing problem precluding the use of magnetic refrigeration systems at elevated temperatures has been the absence of suitable magnetic refrigerant materials and formulations which have a sufficiently large magnetocaloric effect at the elevated temperatures.
There also exists a long standing need for magnetic refrigeration systems that are suitable for use at elevated temperatures, for example, greater than about 100.degree. K, such as 300.degree. K and above.
There also exists an environmental need to eliminate the use of chlorofluorocarbon based heat transfer fluids currently employed in high temperature, such as 300.degree. K and above, gas cycle systems.
There remains a need for magnetic materials which can be used in magnetic refrigeration processes at or above about 273.degree. K and preferably above about 300.degree. K. The also remains a need for magnetic materials which can increase the operating temperature range of magnetic refrigeration processes and also decrease the magnetic field requirements for magnetic refrigeration processes.
Still further, there is a need for nanocomposite nanocrystalline particles that permit low cost, clean, and optionally dry micron and submicron polymeric composite particles that can be selected for use in a magnetic liquid formulation, and utilized as an active component in magnetic refrigeration heat transfer fluids.
Another long standing problem in the field of magnetic refrigeration has been the absence of economical magnetic refrigerant compositions and processes which are capable of functioning efficiently at about ambient temperatures, for example, above 273.degree. K (0.degree. C.), room temperature, for example, about 25.degree. C., and above.
A solution to the above mentioned and related problems has been unexpectedly found and provides for a superior magnetic refrigeration material that enable magnetic refrigeration processes at substantially elevated temperatures compared to those known in the art, for example, at about 0.degree. C. and above, wherein the material has a large magnetic moment, is easily oriented in low magnetic fields and has an ordering temperature at about 0.degree. C. or above, as illustrated herein.